During production of oil from an underground geological rock formation, it is fairly common for paraffin, a naturally occurring constituent of crude oil, to precipitate. Where the wax deposition occures depends on several factors, but is mainly influenced by temperature. There is a distinct tendency, however, for the wax to precipitate in upper portions of the production tubing string. Such precipitation progressively decreases oil production, and eventually plugs the production tubing, stopping oil production entirely.
Removal of wax from a production tubing string is generally accomplished by one of severa methods: mechanical removal (scraping or cutting); use of solvents to dissolve the wax; use of chemical dispersants; and the use of heat to melt the wax. A common practice in wells which use a sucker rod pump to raise oil to the earth's surface is to circulate a hot dewaxing fluid, usually oil, in the well. This is often done by pumping the dewaxing liquid down the annulus, through the pump, and back up the production tubing to the surface. Alternatively, the pump may be unseated, and hot fluid may be pumped down the tubing, around the pump, and back up the annulus to the surface.
Use of such methods in low pressure reservoirs (where wax problems are often most severe) is particularly unsatisfactory. The formation may not be capable of supporting the hydrostatic pressure of a full column of oil in the annulus. In such cases, the hot dewaxing oil is lost into the reservoir and often carries with it wax which has been melted from the production tubing. It is not uncommon for 300 barrels or more of such hot oil to be lost into the reservoir together with melted wax during such dewaxing. The wax removed from the production tubing ends to precipitate in the pores of the rock formation if the reservoir temperature is low enough, an after repeated hot oil treatments, there can be very substantial loss of production. Removal of the wax deposited in the formation can be extremely difficult and costly. Production is also significantly imparied since periods of up to 15 days may be required to pump the fluid originally lost in the hot oil treatment, from the reservoir.